Comets show a diversity in composition, with a factor of 10-100 variability in the volatile abundance [1]}. However, in general this diversity does not appear correlated with the dynamical category. An exception is for CO, whose abundance relative to water appears depleted in Jupiter family comets (JFC), by a factor \(\sim \) 4 in average compared to Oort Cloud comets [2]}. As we show in Fig. REF , the CO depletion in JFC compared to other comets is also visible if expressed as a specific CO production rate, i.e., the production rate per unit area \(Q_{\rm CO}/(\pi D^2/4)\) , where \(D\)  is the equivalent diameter. The top panel of Fig. REF  shows this specific production rate, multiplied by \(r_h^2\) , as a function of the heliocentric distance \(r_h\)  of the measurements. As demonstrated for C/1996 B2 Hyakutake
and C/1995 O1 Hale-Bopp (96B2 and H.B. in the figure), the scaling by \(r_h^2\)  corrects to first order for the distance effects and allows comparison of measurements at different distances. In the bottom panel of Fig. REF , this distance-corrected specific CO production rate is shown as a function of \(D\) . In both panels of Fig. REF , JFCs clearly appear CO-depleted with respect to Oort-cloud comets (OCCs).
As JFCs are thought to originate from the transneptunian region, in particular the Scattered Disk [3]}, [4]} and most of them have diameters \(<\)  5 km, our calculation that only Kuiper belt bodies larger than 4 km can retain CO over the
age of the Solar System may provide a natural explanation to this behaviour. Interestingly, the observed cumulative size distribution of JFC may show an excess of comets with radii 3-6 km [5]}, similar to the above number, and that could account for the diversity of CO abundances within JFCs, although statistics are not sufficient to discern a \(Q_{\rm CO}\)  vs \(D\)  trend within the JFC group. While Fig. REF  is reassuringly consistent with our sublimation calculations for the Kuiper Belt, we note the following
two caveats: (i) the lack of \(>\)  5 km JFCs does not allow us to check our prediction that those objects would be less volatile-depleted (ii) the low CO production rate of JFCs may also be related to their repeated perihelion passages on their current orbits. We note finally that, with the notable exception of 29P/Schwassmann–
Wachmann 1, Centaurs, which are dynamically associated with the Scattered Disk and JFC, also appear CO-depleted compared to Oort-cloud comets [6]} in spite of their large \(\sim \) 100
km size. This is a probable consequence of increased outgassing over their \(10^6\) -\(10^7\)  years lifetime orbits at giant planet heliocentric distances.
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